# CS488 - Introduction to Computer Graphics - Lecture 22

1. Exam: April 16th, 2008, 09.00, RCH???

# Colour

## Light

Almost all the effects are effects of energy

• Coherence is not an issue

Light is both a particle and a wave

#### Geometry

Light travels in straight lines

• except when it doesn't
• Then we re-define straight line'.

Light travels fast

• about one foot per nansecond
• with respect to what
• electrons
• humans

#### Particles

The easiest way to think about light.

Particles have a property

• wavelength (\lambda) or frequency (\nu)
• perceived differently depending on this property
• we call the difference colour'

#### Waves

Wavelength' suggests waves.

Actually two competing theories

• corpuscular theory (Newton)
• wave theory (Huyghens)

Two hundred years ago the wave theory won.

• Young's experiment
• depends on coherence
• Maxwell put the icing on the cake

One hundred years ago a compromise solution was found

## Colour

Colour is the first application of linear algebra.

Grassmann: We don't know what colour is but we know it has certain properties. Let's build a formal system with the properties and see what comes out.

1. Focus on matching
• Two colours are the same' if the look the same'.
• Build colours by adding - in the sense of adding light.
3. Observe linearity
• Changing intensity
4. How many primaries?
• one
• two
• three
5. You need subtraction

So we now have a formal model of colour

1. Choose three standard primaries
2. Express a colour in terms of the primaries
3. You can reproduce it exactly

Expressing a colour in terms of the primaries

1. The colour matching experiment

In practice, the colour gamut' is an issue

## Models of Colour in Computer Graphics

Have an output device with three primaries

1. Express the colour in terms of those primaries
2. You need to do a change of basis, which you know well how to do.
3. But first you need to calibrate the device, which you may not know how to do.